In nitride based semiconductor materials and devices, including visible and ultraviolet (UV) light emitting diodes (LEDs), polarization effects play a dominant role causing strong built-in fields and spatial separation of electrons and holes. These polarization effects can negatively impact the performance of nitride-based visible and ultraviolet light emitting diodes. For example, FIGS. 1A-1C show illustrative band diagrams of a positive-intrinsic-negative (p-i-n) quantum well structure according to the prior art. In particular, FIG. 1A shows a band diagram of the structure without external bias and illumination; FIG. 1B shows a band diagram of the structure with the p-i-n field compensated by external bias; and FIG. 1C shows a band diagram of the structure with the total electric field compensated by external bias and intense optical excitation.
Polarization effects were evaluated for illustrative aluminum indium gallium nitride-based (AlxInyGa1-x-yN-based) multiple quantum well (MQW) structures. The MQW structures comprise an Al molar fraction in the quantum wells and barrier layers close to 20% and 40%, respectively, and In content in both the quantum wells and barriers of approximately 2% and 1%, respectively. The MQW structures comprise a total of three wells, each of which is two to four nanometers thick, separated by four five nanometer thick barriers.
Calculations indicated that the barriers and wells undergo tensions of 0.815% and 0.314%, respectively. These tensions correspond to piezoelectric charges at interfaces induced by this mismatch of −0.0484 coulombs per meter squared (C/m2) for the well and −0.0134 C/m2 for the barrier. The polarization charge was calculated as −0.041 C/m2 and −0.049 C/m2 for the wells and barriers, respectively. The total electric field in the well for an alternating sequence of barriers and wells was found to be 1.2 Megavolts per centimeter (MV/cm). About fifty percent of the field was due to piezoelectric effect and the remaining fifty percent was caused by spontaneous polarization, both having the same direction. This corresponds to a 0.12 eV band bending in a one nanometer wide quantum well. Such band bending precludes using wide quantum wells in deep UV LEDs, which decreases the overall LED efficiency by limiting the MQW design optimization to very narrow (i.e., one to two nanometer thick) quantum wells.